A magnet is a material or object that produces a magnetic field. A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops.
Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic. These include iron, nickel, cobalt, some alloys of rare earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically soft materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically hard materials, which do tend to stay magnetized. Permanent magnets are made from hard ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. Hard materials have high coercivity, whereas soft materials have low coercivity.
For certain applications, magnet operating temperatures are high, such as 100-300° C. Known magnets that are suitable for such applications employ rare-earth elements, which can have significant cost and supply fluctuations.
What is desired is magnets with energy densities equal to high-temperature grade NdFeB magnets at lower cost. Preferably, the magnets have higher energy density than alnico, ferrites, and SmCo magnets, and if possible would be based on iron which is inexpensive. Generally, there is a real need to improve existing commercial and research approaches to magnets.
For example, NdFeB magnets consist of needle-shaped domains of Nd2Fe14B doped with Dy or Tb. These magnets have the highest energy product of 52 MG.Oe available in any commercial magnet and are described in Herbst, Rev. Mod. Phys. 1981, 63, 819. The main disadvantages are the high cost of Nd, Dy and/or Tb and supply disruptions of these elements, and low Curie temperatures that restrict operation to below 80-200° C. They are sold as sintered grains of Nd2Fe14B or Nd2Fe14B needles bonded in a polymer matrix.
SmCo magnets have very high coercivity and can operate up to 350° C. The main disadvantages are the cost of Sm and Co, and reduced energy product as compared to NdFeB magnets.
Alnico magnets consist of FeCo rod-shaped domains in an Al and Ni matrix as described in Luborsky, J. Appl. Phys. 1966, 3, 1091. Such magnets have energy products of 5-9 MG.Oe, which are much lower than magnets with rare-earth elements.
Lodex magnets were created in the 1950s and consist of Fe whiskers that are electroplated in Hg and then aligned and embedded in lead. They have energy products of 5 MG.Oe and are described in Falk, J. Appl. Phys. 1966, 37, 1108.
Magnetic recording tapes consist of anisotropic metal particles consisting of Fe or FeCo in a binder as described in Hisano et al., Magn. Magn. Mater. 1998, 190, 371. The particles are made from Fe2O3 or FeOOH, coated in Al2O3 and Y2O3, where the iron oxides are reduced to iron. In earlier incarnations of magnetic tapes, the magnetic recording particles consisted of alumina-coated iron particles. A flexible matrix, such as the ones used in tapes, would deform during use of a magnet in a motor.
Exchange-spring magnets consist of hard and soft magnetic phases adjacent to one another. The hard magnet provides coercivity and the soft magnet provides a high magnetic moment. Examples of these materials are described in Rui et al., Magn. Magn. Mater. 2008, 320, 2576 and in Zeng et al., Nature 2002, 420, 395. These materials are hard to fabricate and have round magnetic domains.
U.S. Pat. Nos. 5,591,535 and 6,506,264 describe a process for forming iron nanorods coated with alumina and a rare earth oxide. US Patent App. Pub. No. 2012/0244356 describes Fe16N2 rod-shaped particles for use in magnetic recording and magnets. This publication does not teach how to make the particles into a magnet.
FePt or CoPt nanorods can be formed through a wet chemical process. An example of this is described in Chen et al., J. Am. Chem. Soc. 2007, 129, 6348. The nanorods can be assembled into aligned arrays under a magnetic field.
Magnetic nanoparticles have been formed in carbon nanotube arrays. An example of this is described in Shi et al., J. Appl. Phys. 2008, 104, 034307.
In view of the state of the art discussed above, there remains a significant commercial need to improve magnet compositions/composites, methods for making them, and systems that incorporate these magnets.